DAMAGE INSPECTION AND EVALUATION IN THE WHOLE VIEW FIELD USING LASER

Transcription

1 DAMAGE INSPECTION AND EVALUATION IN THE WHOLE VIEW FIELD USING LASER A. Kato and T. A. Moe Department of Mechanical Engineering Chubu University Kasugai, Aichi , Japan ABSTRACT In this study, we investigated a method to evaluate fatigue damage of steels in the whole view field without contact using laser. In the earlier stage of fatigue, slipbands are produced on a metal surface and the slipband density increases with progress of fatigue damage. If a laser light illuminates the specimen surface, diffusion pattern of the reflected light from the specimen surface changes due to surface change. Using this method, we are able to detect fatigue damage based on diffusion pattern change due to surface property change caused by slipbands. We tried three types of optical setups to detect fatigue damaged area in the whole view field of the observing area with CCD camera and evaluate fatigue damage. The diffusion pattern change accompanied by the loading cycles was observed under constant stress amplitude with the three optical setups. We investigated the possibility of detecting fatigue area and evaluating fatigue damage using the optical setups. The results showed that there is a possibility of detecting fatigue damaged zone and evaluating fatigue damage by the optical setup using slit beam of laser light Introduction In the case of fatigue in mild steels, slipband firstly appears on the surface of the test specimen and surface condition changes as slipband density intensifies accompanied by increase of number of loading cycles. In our laboratory, we investigated the methods on evaluation of fatigue damage as well as fatigue life estimation by detecting change in surface condition, which can be seen by observing change in diffusion pattern while laser spot beam is applied to the place where fatigue damage occurred. We can detect and evaluate fatigue damage at the early stage of fatigue before initiation of small cracks. This method can be applied if we know fatigue damage zone but it is firstly necessary to find out the fatigue damage zone unless we know that zone. It is necessary to do damage evaluation after detecting the fatigue damage spot. In our study, we investigated about the evaluation methods and detection of fatigue damage zone of the whole observatory part concerning fatigue of the same steel specimen. It is the most convenient making both detection of damage spot and evaluation with one method, then we also tried about the possibility of this method. We observed the change in diffusion pattern from the surface of the test specimen in the case of receiving fatigue damage with CCD camera by using the optical system that spot beam, slit beam and collimated beam of laser are applied to the surface of the test specimen and scattered light is observed. We tried to investigate fatigue damage detection with evaluation method in the whole view field of a camera. Experimental Procedure The material used in this experiment is a structural steel, JIS SS330. It is indicated chemical composition of the material in Table 1 and mechanical properties in Table 2. The specimen used is a strip with thickness of 3.1mm and slight round cut with the radius of 20mm at the central part as shown in Fig.1. The rolling direction of the material is longitudinal direction of the specimen. Machine hardening and residual stress caused by machining process were removed by normalizing (air cool after holding 30 minutes at 600). Then specimen surface was polished with emery paper its particle number is #800 as the final finish. We made the experiment using a fluid servo system fatigue test machine with the repeated stress speed of 20Hz and the sine wave stress cycle maintaining maximum stress fixed and Table 1 Chemical composition of the material (wt.%) C Si Mn P S Table 2 Mechanical properties Yield Strength Tensile Strength Elongation (MPa) (MPa) (%)

2 minimum stress 0. At suitable intervals of repeated stress cycles, stop the test machine and then observation due to the optical system expressed later and measurement of surface roughness were made for the specimen. Surface roughness was measured to longitudinal and perpendicular direction of the specimen at the surface nearby central round cut edge. The experimental setups used in our study are 3 types as shown in Fig.2. All light sources used are He-Ne laser (output wave length = 632.8nm, beam diameter is about 1mm, output power is 8mW). First, Fig.2(a) shows the same optical system as the one we previously used. Laser spot beam illuminates the specimen surface and a diffusion pattern due to the reflected light from the specimen surface is formed on the ground glass placed in front of the specimen as a screen. An image of the diffusion pattern is obtained by a CCD camera and input into PC. The point applied by spot beam is the point on the surface edge close to notch root. Fig. 1 Test specimen Slit beam scan Collimated beam The optical system shown in Fig.2(b) shows laser slit beam illuminates the specimen surface at longitudinal direction and a diffusion pattern due to the reflected light from the specimen surface is observed with a CCD camera. We tried the fatigue damage detection in the observatory area by scanning slit beam on the central part of the specimen. Details will be explained later. (a) Spot beam Specimen Specimen (b) Slit beam (c) Collimated beam Fig. 2 Detection method of fatigue damage The optical system shown in Fig.2(c) shows the parallel light (beam diameter of about 70mm) due to spatial filter and collimator lens illuminates the specimen surface and a speckle pattern image of the specimen surface is obtained by a CCD camera and input into PC. Speckle pattern changes due to unevenness distribution of the surface profile and we can consider that initial surface polished by emery paper and the surface, slipbands broken out due to fatigue have a great difference in unevenness distribution and sense of direction. Surface Condition and Change in Diffusion Pattern Due to Fatigue In the case of stress amplitude σ a =150MPa, change in surface profile diagrams and observatory results of each optical system of Fig.2 due to number of loading cycles N are shown in Fig.3(a), (b), (c) and (d). N=0 is the initial condition. About the surface profile diagrams of (a), we find that as increase in number of loading cycles N and surface roughness becomes gradually larger due to increase in slipband density. (b) is the diffusion pattern of the reflected light from the specimen surface illuminated by spot beam same as to the one used as in the past in our lab [1]. As increase in the number of loading cycles, point of the reflected light broadens from initial stage. In the case of N= , light intensity at the center declines at the moment and diffusion pattern broadens. (c) is the diffusion light distribution of slit beam. It is the diffusion pattern in the case of slit beam applied to the bottom part of the notch root. (d) is an illumination of collimated beam. The part, a dark stripe breaks out in central part in horizontal direction is seemed to occur slipbands, and it can be seen that width of them broadens as increase in number of loading cycles. According to this method, we can consider that fatigue damage part can be detected by observing difference in brightness. If we compare this result to that of (c) of slit beam, it can be thought of that slipbands appear at the central part of the straight-line shape reflected light of the slit beam. In that part, light is scattered and brightness is declined. The length of the part where light is scattered becomes longer with increasing numbers of loading cycles and corresponds to the broadness of slipbands in (d). From these results, reflected light pattern from a spot beam broadens as N increases and fatigue damage progresses as shown in Fig. 3 (b). This shows that fatigue damage evaluation at a point on the specimen is possible. It was found in our previous study that evaluation of fatigue damage and fatigue life estimation is possible with this method. In (d), speckle pattern on the specimen surface illuminated by a collimated beam differs in the areas where slipbands appeared and not appeared and fatigue damaged area is visualized clearly on the camera image. Detection of initiation and expansion of the fatigue damaged zone is possible by this method. It is not easy to evaluate amount of fatigue damage from only these images, however. In (c), reflected light from a slit beam has a cut at the center where slipbands appear, comparing the images in (d). This is because that that diffusion of reflected light from the specimen surface occurs at position where slipbands appear and

3 (1) N = 0 10mm 10mm 5mm (2) N 4 = 3 10 (3) N 4 = (a) Surface profile diagrams (b) Spot beam (c) Slit beam (d) Collimated beam Fig. 3 Change in surface profile and diffusion pattern by fatigue (σ a =150MPa) light distribution broadens and the maximum brightness of the reflected light decreases. Thus, brightness at the center decreases by diffusion and line cut appears shown in (c). Diffusion at the center with slipband on the surface seems to differ at different stage of fatigue. Analytical method for evaluating light intensity distribution was considered. We investigated this slit beam method for detection and evaluation of fatigue damage next. Fatigue Damage Evaluation by Slit Beam We observed diffusion pattern change by slit beam of laser. The experimental setup used is shown in Fig. 4. The spot beam is changed to slit beam through a cylindrical lens. Slit beam illuminates the specimen surface at seven locations scanning the slit beam shown in Fig. 2 (a). Reflected light from the specimen forms reflection pattern on the ground glass. Reflected light intensities are captured by a CCD camera as digital images and analysed the image on a PC. Figure 5 shows examples of the observed diffusion pattern. The figures are images after superimposing the each image from seven different slit beam. In the figure, (b) is the initial diffusion pattern. There is no line cut in each reflected light line in (b). In (c), the reflected light lines are cut very slightly at center. Slipband appears firstly at the center from the notch root as very thin line in the specimen. Surface property changes by slipband. Reflected light diffuses at the position where surface change occurred by slipband caused by fatigue. The slight cut of the line in (c) shows the location where slipband started to appear. As number of loading cycles increases, the light cuts become longer Specimen CCD camera Ground glass Mirror To PC Cylindrical lens Mirror ND filter He-Ne laser Fig. 4 Experimental system Micro-rotary stage Fig. 4 Optical setup for slit beam illumination

4 (a) Position of slit beam (b) N = 0 (c) N = (d) N = (e) N = Fig. 5 Reflected light pattern of slit beam (σ a =150MPa) shown in (d) and (e) and we can know the fatigue damaged zone expands. In Fig. 5, it is found that line cut of the reflected light of slit beam expresses diffusion of laser light reflected from the specimen surface and the location of the light line cut corresponds to the location of slipband caused by fatigue damage on the specimen. We can find very clearly initiation and location of fatigue damage of the specimen from the images shown in Fig. 5 and these images are very useful to visualize fatigue damage on the specimen. Detection of the fatigue damaged zone is possible observing this kind of image. It is more useful, if it is possible to evaluate fatigue damage based on these observations. We considered a method to evaluate amount of fatigue damage from this observation result next Figure 6 shows gray level distribution of reflected light from the specimen surface on horizontal direction vertical to reflected light line as shown in the figure. Gray level distribution at the fatigue damaged zone is shown for different number of loading cycles. The maximum gray level is high and the distribution has very peaky distribution at the initial situation (N=0). The maximum gray level lowers and gray level distribution expands as N increases. It is clear from the figure that gray level distribution of the reflected light at fatigue damaged zone changes as fatigue damage increases. We considered a method to evaluate characteristics of the gray level distribution. An example of the gray level distribution at diffused location of the reflected light is shown in Fig. 7 (a). Light intensity far from the center should be zero, but the image from the CCD camera has small value. And gray levels at right and left side far from the center are slightly different. This seems to be because that the laser slit beam does not illuminate on the specimen perfectly vertical on the surface, but has a small angle from the vertical and reflection is slightly different at right and left side of the illuminated position. The base gray level was obtained by linear approximation using gray levels far from the center and moves base line to zero position (c). Horizontal coordinate, x c, of the centroid of the gray level distribution was obtained and gray level distribution is approximated with the following equation, g b x x = ae c (1) where the coefficients a and b in the equation are obtained from the gray level values with the least squares method. Obtained curve from the experimental data is shown in the figure (c). The figure shows the curve fit very well with the experimental data. The coefficients a and b express characteristics of the gray level distribution. Relation between the coefficients a and b with number of loading cycles is shown in Figure 9 (a). The figure shows that both coefficients a and b decrease as number of loading cycles increases. In Eq. (1), coefficient a is the maximum value of g at the center. Width w h where g takes half of the maximum value is expressed with the following equation Fig. 8. ln 2 w h = (2) b The parameter w h expresses width of the gray level distribution. Relation between w h and N is shown in Fig. 9. From the figure, it is found that w h increases clearly as N increases. The specimen fractured soon after the last observation at N= The parameter w h increases with N until fracture. We can conclude that w h changes with progress of fatigue and it is possible to N=0 N=510 4 N= N= Fig. 6 Comparison of grey level distribution Diffusion pattern by slit beam

5 use this parameter to express amount of fatigue damage. Conclusions In this study, we investigated a method to detect and evaluate fatigue damage for steel specimen using laser. We tried three kinds of optical setups illuminating laser light with (a) spot beam, (b) slit beam and (c) collimated beam. It was found that the method using slit beam has a possibility of both detection and evaluation of fatigue damage in the whole view field observing with a CCD camera In these optical setups, We observed reflection light pattern of laser slit beam illuminated on the specimen surface. We can find fatigue damaged zone in the whole view field scanning the slit beam on the specimen surface and succeeded to visualize fatigue damaged zone with an image of the reflected light pattern. It was clarified that gray level distribution of reflected light at fatigue damaged zone changes with number of loading cycles. We considered a method to evaluate the characteristics of the gray level distribution. We derived a parameter to characterize the gray level distribution of the laser diffusion pattern and it was found that the parameter changes clearly with progress of fatigue damage. We can use the parameter to evaluate fatigue damage. From this study, it was found that we can use the method of laser light reflection from specimen surface with slit beam to visualize the fatigue damaged zone and detect easily fatigue damage initiation and expansion of the area. There is a possibility to use the method we developed in this study to detect fatigue damaged position and also evaluate amount of fatigue damage with one optical setup. This seams to be very effective for actual application. Acknowledgments This research was supported by a grant from the High-Tech Research Center Establishment Project from Ministry of Education, Culture, Sports, Science and Technology, Japan. (a) Grey level distribution (b) Base gray level adjustment References 1. Kato, A. and Sano, M., Fatigue Life Estimation Using Diffusion of Laser, Advances in Nondestructive Evaluation, Trans Tech Publications, (2004) 2. Kato, A., Kawamura, M., and Nakaya, I., Damage Monitoring of Metal Materials by Laser Speckle Assisted by Image Processing Techniques, JSME Int. J., Ser. A, 38 (2), (1995). g = ae b x x c x c (c) Curve fitting with an exponential function Fig. 7 Procedure for evaluation of gray level distribution

6 g a 0.5a g= ae b x x c a Fractured at N= b 0 w h x x c Fig. 8 Calculation of parameter w h w h N (10 4 ) (a) Coefficient a and b Fractured at N= N (10 4 ) (b) Change of the parameter w h Fig. 9 Change of parameters evaluating gray level distribution in diffusion pattern